Demand for portable electronic devices is increasing each year. Example portable electronic devices include: laptop computers, personal data assistants (PDAs), cellular telephones, and electronic pagers. Portable electronic devices place high importance on total weight, size, and battery life for the devices.
Most portable electronic devices employ rechargeable batteries. Commonly used rechargeable batteries include Nickel-Cadmium (NiCad), Nickel-Metal-Hydride (NiMHi), Lithium-Ion (Li-Ion), and Lithium-Polymer based technologies. Charger circuits are commonly available for each of these types of battery technologies. Each charger circuit often includes a shunt regulator to control the amount of charge that is delivered to the battery.
An example shunt regulator charging system is shown in FIG. 3. As shown in FIG. 3, the shunt regulator charging system (300) includes a power source (301), a shunt regulator (302), and a load circuit (303). The power source (301) includes a voltage source (VS) and a source resistance (RS). The shunt regulator (302) includes a NMOS transistor (MN), a PMOS transistor (MP), two amplifiers (AMP1, AMP2), three resistors (R31–R33), and a switch control circuit (SCON). The load circuit (303) includes a rechargeable battery (BATT).
In operation the power source provides a charging current (ICHG) to the rechargeable battery through source resistance RS, PMOS transistor MP, and resistor R33. Resistor R33 converts the charging current (ICHG) into a voltage (VSNS), which is used by amplifier AMP2 and switch control circuit SCON to control the activation of transistor MP. PMOS transistor MP is activated during normal charging operations. Resistors R31 and R32 form a voltage divider that provides a feedback signal (VFB) to amplifier AMP1. Amplifier AMP1 compares the feedback signal (VFB) to a reference voltage (VREF) and provides a control signal (SHCON) to transistor MN. Transistor MN, amplifier AMP1, and resistors R31–R32 together operate as a shunt regulator that regulates the input voltage (VIN). The shunt regulator provides safe charging of rechargeable battery BATT by limiting the charging voltage (input voltage) similar to a zener diode.
Many portable power applications use a DC—DC converter to either step up or down the battery voltage for use with particular circuits. For example, video display circuits typically require an operating voltage that exceeds the battery voltage. In a DC—DC converter, the output voltage is provided to a load circuit by varying the conduction time that is associated with a controlled device. Example controlled devices include transistors, gate-turn-on (GTO devices), thyristors, diodes, as well as others. The frequency, duty cycle, and conduction time of the controlled device is varied to adjust the average output voltage to the load. Typical DC—DC converters are operated with some sort of oscillator circuit that provides a clock signal. The output voltage of the converter is also determined by the oscillation frequency associated with the clock signal.